Communication systems are known to support wireless and wireline communications between wireless and/or wireline communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, for example, a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, or home entertainment equipment, communicates directly or indirectly with other wireless communication devices. For direct communications, also known as point-to-point communications, the participating wireless communication devices tune their receivers and transmitters to the same channel, or channels, and communicate over that channel(s). Each channel may utilize one or more of the plurality of radio frequency (RF) carriers of the wireless communication system. For indirect wireless communications, each wireless communication device communicates directly with an associated base station, for example, for cellular services, and/or an associated access point, for example, for an in-home or in-building wireless network, via an assigned channel or channels. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network, via Internet, and/or via some other wide area network.
In order for each wireless communication device to participate in wireless communication, it utilizes a built-in radio transceiver, which comprises a receiver and a transmitter, or it is coupled to an associated radio transceiver, for example, a station for in-home and/or in-building wireless communication networks, or a RF modem. The transmitter converts data into RF signals by modulating the data in accordance with the particular wireless communication standard to produce a baseband signal. The baseband signal is mixed with a local oscillator signal in one or more intermediate frequency stages to produce the RF signal. The radio receiver generally includes an antenna section, a filtering section, a low noise amplifier, an intermediate frequency (IF) stage, and a demodulator. The antenna section receives the RF signal and provides it to the filtering section, which, in turn, passes a filtered RF signal to the low noise amplifier. The low noise amplifier amplifies the filtered RF signal and provides an amplified RF signal to the IF stage. The IF stage steps down the frequency of the amplified RF signal to an intermediate frequency or to baseband. The IF stage provides the IF signal or baseband signal to the demodulator, which recaptures the data in accordance with the demodulation protocol.
For the demodulator to accurately recover data from the IF signals or the baseband signals, unwanted direct current (DC) offsets must be overcome. One source of DC offsets in the demodulated output of a frequency modulated (FM) system is when the local oscillator of a transmitting radio generates a signal with a slightly different frequency than the frequency of the signal produced by the receiving radio local oscillator. To correct for the DC offset, a demodulator in a radio receiver includes a DC offset detection circuit and DC offset compensation circuit. The DC offset detection circuit indicates the level of DC offset due to frequency mismatch. The DC compensation circuit removes the DC offset indicated by the DC offset detection circuit from the demodulated IF signals or baseband signals before data extraction. The DC offset due to frequency mismatch can adversely affect the data extracted from the IF or baseband signals.
For example, Bluetooth utilizes a 64-bit synchronization (SYNC) word, which comprises a predefined bit sequence. The 64-bit synchronization (SYNC) word is utilized for identifying devices that want to communicate with each other. Hence, devices wishing to communicate with each other must identify the 64-bit synchronization (SYNC) word via a correlation process. After successful correlation, communication may take place among the Bluetooth devices. The DC offset sometimes interferes with identifying the 64-bit synchronization (SYNC) word, and as a result, the 64-bit synchronization (SYNC) word is not correlated. As an example, if a synchronization threshold is set at 56 bits for a 64-bit synchronization (SYNC) word and the first 6 bits are misidentified due to the DC offset and there are three other bit errors in the remainder of the 64-bit synchronization (SYNC) word, then the synchronization pattern will be missed.
The presence of the DC offset may require the use of circuitry in order to compensate for the frequency difference. This compensation circuitry may require additional area in an integrated circuit (IC) and/or may require additional power during receiver operation.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.